Configure PAM for the login service you're using. For example, for ssh, add this to /etc/pam.d/sshd:
Then create a file called /etc/sshd.deny, and list that user name.
auth required /lib/security/pam_listfile.so onerr=fail item=user \
sense=deny file=/etc/sshd.deny
Then create a file called /etc/sshd.deny, and list that user name.

Charles Darke | 3 May 2008